The present disclosure relates to technology for non-volatile storage.
Semiconductor memory has become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in personal navigation devices, cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Some EEPROMs or flash memory devices have a configuration referred to as a NAND configuration in which memory cells are grouped as NAND strings with each NAND string associated with a bit line. One type of NAND memory array is a two-dimensional array. Another type of NAND memory array is a three-dimensional array. One 3D NAND stacked memory device is sometimes referred to as a Bit Cost Scalable (BiCS) architecture.
In a 3D NAND BiCS architecture, a 3D NAND stacked memory device can be formed from an array of alternating conductor and insulator layers. A memory hole is drilled in the layers to define many memory layers simultaneously. A NAND string is then formed by filling the memory hole with appropriate materials. A straight NAND string extends in one memory hole, while a pipe- or U-shaped NAND string (P-BiCS) includes a pair of vertical columns of memory cells which extend in two memory holes and which are joined by a pipe connection. Control gates of the memory cells are provided by the conductor layers.
Another example of non-volatile memory uses variable resistance memory elements that may be set to either low or high resistance states. Upon application of sufficient voltage, current, or other stimulus, the variable resistance memory element switches to a stable low-resistance state, which is sometimes referred to as SETTING the device. This resistivity-switching is reversible such that subsequent application of an appropriate voltage, current, or other stimulus can serve to return the reversible resistivity-switching material to a stable high-resistance state, which is sometimes referred to as RESETTING the device. This conversion can be repeated many times.
The variable resistance memory elements may be in a high resistance state when first manufactured. This may be referred to as the “virgin state.” In the virgin state, the resistance could be even higher than for the RESET state. The term “FORMING” is sometimes used to describe putting the variable resistance memory elements into a lower resistance state for the first time. For some memory elements, the FORMING operation requires a higher voltage than the SET and/or RESET operations.
3D memory arrays having variable resistance memory elements have been proposed.
In one possible architecture, word lines extend horizontally and bit lines extend vertically. There a multiple levels of the word lines, hence multiple levels of memory elements. Each memory element is located between one of the vertical bit lines and one of the horizontal word lines. During operation, some of the memory cells are selected for the SET, RESET, or FORM operation, while others are unselected.